The present invention relates to a laser scanning optical system having a deflecting device for dynamically deflecting a laser beam emitted from a laser source, and a mirror to introduce the deflected laser beam in a predetermined direction.
Recently known is a printer employing a so-called electrophotographic image forming process such as a laser beam printer, wherein a uniformly charged photoconductive material, provided on a photoconductive drum, is exposed to a scanning laser beam which in ON/OFF modulated, based on an image data, to form an electrostatic latent image. A hard copy of the image data is obtained through a copying process comprising development, transferring, and fixing steps.
FIG. 3 shows an example of the laser beam printer comprising a charging unit A, exposing unit B, development unit C, transfer unit D, cleaner unit E and discharging unit F disposed around a photoconductive drum 1 in the rotational direction thereof. The surface of the photoconductive drum 1 is formed from an electrostatic photoconductive material.
The photoconductive material on the surface of the photoconductive drum 1, uniformly charged at the charging unit A, is exposed to a scanning laser beam carrying image data (ON/OFF modulated in accordance with the image data) at the exposing unit B to form a latent image on the photoconductive material. Toner is stuck to the latent image at the development unit C for forming a toner image, and the toner image is transferred onto a recording sheet P which is fed synchronously with the rotation of the photoconductive drum 1 at the transfer unit D. The toner image transferred onto the recording paper P is then fused by being heated/pressed at a fixing unit G. Thus the toner image is fixed on the recording sheet P. Then the recording sheet P carrying the fixed toner image is discharged from the printer.
In the laser beam printer, the transfer unit D is disposed below the photoconductive drum 1. A recording sheet feed path is defined in the substantially horizontal direction on the right and left sides of the transfer unit D. A scanning optical system 10 as the exposing unit B is disposed above the photoconductive drum 1.
In the scanning optical system 10, a laser beam LB, emitted from a semiconductor laser, s repeatedly deflected by a polygon mirror 12 as a deflecting device. The repeatedly reflected laser beam (scanning laser beam) is imaged by an f.theta. lens 13 so that the scanning speed thereof, on the circumferential surface of the photoconductive drum 1, becomes an equal speed in proportion to a deflected angle. The light path thereof is reflected by a mirror 16 so that the laser beam is directed to the photoconductive drum 1. Main scanning of the laser beam, emitted from the scanning optical system 10, is executed in the axial direction of the photoconductive drum 1. Auxiliary scanning in the rotational direction of the photoconductive drum 1 is synchronous with the main scanning by the laser beam.
In this scanning optical system 10, each optical component must be installed with a high accuracy, and generally accommodated in a sealed cabinet in order to prevent the invasion and deposition of dust and the like.
Note that, the length of the light path of the laser beam from the polygon mirror 12 to the photoconductive drum 1 is preferably as short as possible to make the apparatus compact as a whole, whereas the length is preferably as long as possible in view of the aberration of the lens system and to keep the cost as low as possible.
Generally, the scanning optical system 10 is disposed above the photoconductive drum 1 as above, and the fixing unit G employs the heat roller fixing unit in which an unfixed toner image is heated and pressed with a pair of rollers to cause toner to be heated and adhered onto the recording sheet by being fused. In such a case, the scanning optical system 10 is disposed above the heat roll fixing unit, and is partially heated by the heat of the heat roll fixing unit. Thus, a problem arises in that moisture is condensed on the optical components in the scanning optical system 10 particularly in the environment of low temperature and high humidity.
More specifically, in the illustrated arrangement, the vicinity of the polygon mirror 12 is heated and the temperature of the air around the polygon mirror 12 is raised, and thus a temperature difference is caused between opposite sides of the casing (between the polygon mirror 12 side and the mirror 16 side). As a result, the rotation of the polygon mirror 12 causes the heated air around the polygon mirror 12 to reach the region of the mirror 16 on the other side, where the heated air is cooled on the surface of the mirror 16 and moisture is condensed on the mirror 16. In this case, even if the apparatus operates normally, a scanning laser beam does not reach the photoconductive drum 1 in good condition. Thus, a formed image becomes obscure on an image is not formed depending upon the degree of the moisture condensation.
Further, although not shown in FIG. 3, in the scanning optical system 10, a scanning laser beam at the ends of a scanning area, which does not contribute to the image formation, is reflected by a mirror (laser beam position detecting mirror) and introduced to a light sensor. The laser beam is modulated using a laser beam detecting signal from the light sensor as a trigger signal. When, however, moisture is condensed on the laser position detecting mirror, no image can be formed because the position of the scanning laser beam cannot be detected.
Further, as shown by the two-dotted lines in the figure, when the portion of the scanning optical system is swung about an axis X provided in the polygon mirror 12 side (with the high temperature side downward, low temperature side upward) to load a recording sheet or carry out maintenance, heated air instantaneously moves (rises) to the mirror 16 side, thereby moisture is heavily condensed on the mirror 16.
Generally, in order to prevent the moisture condensation, a method of heating components, on which moisture tends to be condensed by providing heating means therewith has been devised. In such a construction, however, a problem arises in that the structure thereof becomes complex because a power supply, wirings and temperature control are needed, and thus cost is increased.